The invention relates to a method for determining the moment when, in the course of an operation of freezing a biological body of a predetermined shape, the freezing becomes complete.
The invention also relates to an automatic apparatus for monitoring the impedance of biological bodies, more especially that impedance produced in applying the aforesaid method.
The apparatus according to the invention, which is called hereinafter an "impedometer" is particularly valuable for resolving the difficulty which exists in determining the state of complete congelation of a mass of tissue which it is proposed to remove by the surgical technique based upon cryonecrosis.
An apparatus of this type is designed to be integrated into a complete cryosurgical assembly thus greatly facilitating the clinical use of the latter.
In point of fact, the object of cryosurgery is to destroy all the cells of unhealthy tissue while sparing the neighboring healthy tissue to the maximum extent.
The achievement of this result implies, as a preliminary that after having estimated the substantially exact limits of the unhealthy tissue or organ, the determination of the exact conditions of congelation to insure "cryodestruction" as complete as possible within the "target" space. By "target" space is meant the whole of the unhealthy cellular mass which it has been decided to destroy.
Now, when a certain amount of heat is extracted from a biological system, there is a change of phase or change of state which converts the freezable water into ice and has the result of "extracting" from the cell the water of solvation and the "structural" water, in particular the membranous water. Considering that in an aqueous system, water crystallizes first in the form of pure ice, dehydration occurs. Since the stability of a biological system is dependent on the maintenance of an exact concentration of aqueous solutions, the consequences of the loss of the water of solvation incorporated in the crystalline structure of the newly formed ice will be more or less important for the equilibrium of this system; moreover, the concentrated solution will be located between the ice crystals.
It is easy to foresee that the disappearance of "structural" water, necessary for molecular and intermolecular functions will involve changes which are often irreversible, and incompatible with cellular survival. One can then understand the alterations in the membrane systems.
The destructive potential of cryosurgery thus appears to be directly related to the cellular changes caused by the changes of state of the water.
These considerations are known to be confirmed by experiments on tumors produced in animals such as the rat.
It is also known that immediate or short term cryodestruction is all the more complete if the freezing of the unhealthy tissue is effected at a sufficiently high speed. Now, for freezing produced by means of a cryode, operating, for example, by the projection of a jet of liquid nitrogen, 60 to 70% of the maximum freezing effect will be achieved in the first 30 seconds of cooling and 80 to 90% in about five minutes.
Hence there is no advantage in prolonging the freezing action of the cryode beyond five minutes; and besides, beyond this limit, the thermal conductivity of the tissues conjugated with the very low temperatures of the cryode (which can be of the order of -200.degree. C.) would risk causing the freezing of an "envelope" of neighboring healthy tissue which would not be tolerable. It may be considered desirable for this envelope to be limited to 5 to 10 mm in thickness, which signifies that, taking into account the foregoing, the action of the cryode should be interrupted with high accuracy to effect the complete congelation of the unhealthy tissue.
It is known in the art, to determine the state of congelation of the mass of unhealthy tissue to be treated, by the use of probes constituted by thermocouples. These probes have the drawback of only giving a valid indication for the place where they are implanted. On the other hand, they are not reliable since a part of the probe can be in contact with tissue actually frozen while its point is in contact with tissue which is not yet frozen. Now, in such as case, it will nonetheless give the indication of congelation.